The invention relates to a method for forming an insulated air duct, the air duct comprising an inner wall, an outer wall surrounding the inner wall, and a thermal insulation layer between the inner wall and the outer wall. The invention also relates to an insulated air duct made by the method.

Air ducts of buildings are typically installed in the roof of the building, whereby they are concealed in the structures. The air ducts in the roof have to be insulated carefully so that the moisture contained in the air flowing in the ducts cannot con- densate in the exhaust air vent in the winter and in the air inlet duct in the summer, in connection with cooling. The air ducts are normally insulated by insulating chutes which are installed tightly around the air ducts. Insulation of the air ducts is a separate work step which is performed after the actual building of the ducts. The material of the insulation chutes used for insulation is typically mineral wool, cellu- lar plastic or polyester fibre.

Several drawbacks are involved in the insulation of air ducts according to the state of the art. The roof of the building is typically a shallow and narrow space, and working there is difficult. The installation of fluffy mineral wool insulation materials in such a space is difficult and requires the use of special means of protection, such as breathing masks. The insulation of the ducts is particularly difficult in buildings under renovation, in which the thermal insulation materials of the roof have already been installed. Therefore, the installation of the thermal insulation around the air ducts forms, as such, a significant part of the installation costs of the air ducts. The insulation chutes used for installing the air ducts are tubular elements made of insulation material and having a cut extending through the wall of the element. The insulation chute is installed around the tube by slightly opening the cut so that the insulation can be slipped around the air duct. After that, adhesive tape is attached onto the outer surface of the insulation chute so that the butt joint at the cut of the chute becomes tight. In other words, the insulation is not actually fastened to the wall of the air duct, but the insulation chute is a separate element that is tightly clamped around the air duct. It has been found in practice that the tapes may come off over time, so that a gap splits open at the cut of the vent chute, decreasing the insulating capacity. Insulating chutes can also be clamped around the ducts by wires or wire nettings. This involves the risk of compressing the insulation too much, impairing its thermal insulation capacity.

A double wall air duct is known, which has a tubular inner wall and a cylindrical outer wall with a larger diameter surrounding the inner wall. The inner and outer walls consist of hot-dip zinc coated steel sheet with a thickness of 0.5 mm. A layer of glass wool is compressed between the inner and outer walls. A plastic air duct is also known which has a cellular plastic layer with a thickness of 15 mm glued onto its outer surface. Such air ducts are expensive in terms of the manufacturing costs. Patent publication JPH06159586 discloses a tubular air duct with a layer of silicate based thermal insulation attached to its outer surface.

The aim of the invention is to present a method for forming an insulated air duct, and an insulated air duct made by the method, which can eliminate drawbacks related to the state of the art. The aims of the invention are achieved with the method and the air duct characterized by what will be presented in the independent claims. Some advantageous embodiments of the invention will be presented in the dependent claims.

The invention relates to a method for forming an insulated air duct. The air duct comprises an inner wall, an outer wall surrounding the inner wall, and a thermal insulation layer between the inner wall and the outer wall. A casting mould with a first opening, a second opening and a mould surface is used in the method. The mould surface refers to the inner surface of the mould, that is, the surface of the mould wall that forms the boundary surface of the space delimited by the mould. This mould surface has a cross-sectional shape that is substantially constant in the section between the first opening and the second opening. In the method, a prefabricated inner wall and a prefabricated outer wall are used, which are fitted in the casting mould in such a way that the inner wall is surrounded by the outer wall. The inner and outer walls can be fitted in the casting mould in any suitable order. For example, the outer wall can be fitted in the casting mould first, after which the inner wall is fitted inside the outer wall. Alternatively, the outer wall can be fitted around the inner wall first, after which both walls are inserted simultaneously in the casting mould. The inner and outer walls are dimensioned so that they extend from the first opening of the casting mould to the second opening of the casting mould. After both the walls have been inserted in the casting mould, the inner wall is supported by fixing it with respect to the casting mould. The casting mould is preferably supported at its ends; in other words, at the first and second opening of the casting mould. After this, an expanding and curable foamed thermal insulation ma- terial, preferably polyurethane, is dispensed between the inner wall and the outer wall, and allowed to expand and substantially fill the whole space between the inner wall and the outer wall, and to adhere to the inner wall and the outer wall. After curing, a rigid layer of thermal insulation is provided between the inner wall and the outer wall. Finally, the insulated air duct consisting of the inner wall, the outer wall and the thermal insulation layer is removed from the casting mould.

In an advantageous embodiment of the method according to the invention, the inner wall and the outer wall are fitted in the casting mould and the foamed thermal insulation material is dispensed between the inner wall and the outer wall through the first and/or the second opening. Thus, no openings, holes or openable sec- tions are needed at all in the section between the first and second openings on the wall of the casting mould, whereby the mould surface of the casting mould can be a solid, closed surface in the section between the first and second openings. Advantageously, the insulating air duct made by the method and consisting of the inner wall, the outer wall and the thermal insulation layer, is removed from the casting mould via the first or second opening. It has been found that the elongate insulated air duct will come out of the casting mould via the opening when the air duct is subjected to a reasonable amount of force in the longitudinal direction of the casting mould. In the removal step, the frictional force effective on the outer wall of the air duct is so small that it will not crack the outer wall or the thermal in- sulation layer. There is thus no need to open or disassemble the casting mould when removing the air duct, except by opening the first and second openings.

In another advantageous embodiment of the method according to the invention, the outer wall is formed of a thin foil. A hose that is open at both ends is formed of this foil and fitted in the casting mould. Advantageously, the hose is dimensioned so that the perimeter of its cross section is slightly smaller than the perimeter of the cross section of the casting mould. When the foamed thermal insulation material dispensed between the inner wall and the outer wall expands, it forces the outer wall to be pressed against the mould surface of the casting mould. Thus, the outer wall of the thermally insulated air duct assumes the shape of the mould sur- face of the casting mould. When expanding, the hose stretches slightly against the casting mould, wherein a smooth hose surface is obtained. Preferably, the outer wall is formed of a thin metal foil, advantageously aluminium foil.

In a third advantageous embodiment of the method according to the invention, at least one strap, preferably a metal strap, is provided against the inner surface of the outer wall, extending from the first opening to the second opening. In this context, the inner surface of the outer wall refers to the surface of the outer wall facing the inner wall. By means of the straps, the outer wall can be forced against the mould surface of the casting mould, or close to it, whereby an open space is formed between the inner wall and the outer wall, for dispensing the thermal insu- lation material. After the curing of the thermal insulation, the strap constitutes a strip that is resistant to tensile stress in the air duct, right below the outer wall.

In yet another advantageous embodiment of the method according to the invention, the inner wall is formed of a rigid air duct, such as a metal or plastic pipe. A rigid air pipe can be easily fitted inside the casting mould and supported at its ends to be immobile inside the casting mould.

In yet another advantageous embodiment of the method according to the invention, the inner wall is made of a thin foil, preferably a metal or plastic foil. The thickness of the foil can be some hundredth or tenth parts of a millimeter. An inner wall formed of such a foil does not have a sufficient structural rigidity as such, so that a structurally rigid central core is used to support it in the casting mould. In this embodiment, the inner wall is fitted around a separate rigid central core, and the central core is supported to be immobile inside the casting mould. After curing of the thermal insulation material, the central core is removed from inside the inner wall. The central core can be, for example, a pipe with a smooth surface, or a solid bar. The central core can be removed from the inside of the inner wall by pulling or pushing the central core out; that is, in the same way as the finished insulated air duct is removed from the casting mould. Thus, for removing the central core, it will not be necessary to break the inner or outer wall or the thermal insulation layer.

In yet another advantageous embodiment of the method according to the inven- tion, an elongate casting mould with a cylindrical mould surface is used. The inner wall is supported to be immobile substantially in the centre of the casting mould. The length of the elongate casting mould can be, for example, about 3 metres or another suitable length. The insulated air duct according to the invention comprises an inner wall, an outer wall enclosing the inner wall, and a thermal insulation layer between the inner wall and the outer wall. The thermal insulation layer consists of a hard, foamed thermal insulation material, preferably polyurethane, and the thermal insulation layer has an outer surface attached to the outer wall, and an inner surface attached to the inner wall. Preferably, the outer wall consists of a thin metal foil, such as aluminium foil. The thickness of the metal foil can be, for example, 0.1 to 0.2 mm. The thin metal foil improves the thermal insulation capacity and the mechanical strength of the thermal insulation layer but does not significantly increase the weight of the air duct.

In another advantageous embodiment of the air duct according to the invention, the inner surface of the outer wall is provided with at least one strap, preferably a metal strap, extending from the first end to the second end of the air duct. Advantageously, the inner surface of the outer wall is provided with four straps at sub- stantially regular intervals on the perimeter of the inner surface of the outer wall. The straps constitute a strip that is resistant to traction immediately below the outer wall, to which strip pipe fittings can be fixed, for example by screw fastening, to be placed against the outer wall. The straps can thus be used for fastening air ducts to each other. In another advantageous embodiment of the method according to the invention, the inner wall is a rigid air pipe. Such an air pipe has a structural rigidity sufficient to maintain its tubular shape without external support. The rigid thermal insulation layer attached to the outer surface of the inner wall increases the overall rigidity of the air duct further. Alternatively, the inner wall may consist of thin foil. Such foil may have a thickness as small as some hundredth or tenth parts of a millimetre, whereby it does not, as such, have a structural rigidity sufficient to maintain the open, preferably tubular, shape of the inner wall. However, the inner wall is attached, by its outer surface, to the hard and structurally rigid thermal insulation layer, enabling the inner wall to maintain its open shape in use. Preferably, the material of the inner wall is plastic or metal.

In yet another advantageous embodiment of the air duct according to the invention, the inner wall and the outer wall are cylindrical, in other words, they have a circular cross section. Thus, the inner wall is enclosed by a thermal insulation layer with a substantially uniform thickness. However, the inner wall and the outer wall may also have a shape deviating from the cylindrical shape, and the inner wall and the outer wall can have different cross sectional shapes. The method according to the invention has the advantage of making the manufacture of a thermally insulated air pipe significantly simpler and faster, which reduces the manufacturing costs of the air pipe.

The thermally insulated air pipe according to the invention has the advantage that the thermal insulation capacity of the insulation layer is very good. Furthermore, the air pipe is light in weight, which facilitates the installation work. The installation work is also facilitated by the fact that the separate step of installing the thermal insulation is totally eliminated.

In the following, the invention will be described in detail. In the description, refer- ence will be made to the appended drawings, in which

Fig. 1 shows an example of a mould device used in a method according to the invention, seen diagonally from above,

Figs. 2a to 2d show examples of different steps of the method according to the invention in a series of images, Fig. 3a shows an example of a joint between two insulated air ducts according to the invention, in a cross-sectional view, and

Fig. 3b shows an example of a band for use in a joint of an air duct.

Figure 1 shows an example of a mould device used in a method according to the invention, seen diagonally from above. The mould device comprises an elongate casting mould 100 with a cylindrical wall 102. In this context, the inner surface of the wall is called the mould surface 104. The mould surface constitutes the boundary surface of the space delimited by the casting mould. The casting mould is provided with a first opening 106 at its first end and a second opening 108 at its second end. The openings are circular in shape and equal in size. The cross-sec- tion of the hollow inner part of the casting mould is circular over the whole length of the mould, and equal in size with the first and second openings. At the first and second ends, the casting mould is provided with an annular collar 100 extending outwards at a right angle to the plane of the outer surface of the casting mould and having through holes 1 12 for receiving mounting bolts. Moreover, the mould device comprises two circular end plates 1 14 which are equal in shape and equipped with a conical supporting trunnion 1 16 on one surface. The supporting trunnion is fastened at its wider end to one surface of the end plate so that the imaginary central axis of the trunnion extends via the centre of the end plate. Mounting holes 1 18 are provided on the perimeter of the end plate, their placement matching the placement of the mounting holes in the collar 1 10. Furthermore, the end plates are provided with four filling holes 120 placed within an area delimited by the mounting holes, at the corner points of an imaginary square around the supporting trunnion 1 16. The diameter of the filling holes is preferably 22 mm.

The material of the casting mould and the end plates, as well as the supporting trunnions in the end plates, is metal, preferably steel. Casting moulds can be made in different sizes in terms of length and cross-section, for forming insulated air ducts in different sizes. Advantageously, the length of the casting mould is at least 3 metres. The diameter of the mould surface of the casting mould can be, for example, 160, 200, 185, 220, 225, or 260 mm. The size of the end plates 1 14 is determined by the size of the opening 106, 108 of the casting mould. The diameter of the supporting trunnion 1 16 on the end plates is dimensioned so that its butt end, that is, its widest end, is about 0.5 mm smaller than the diameter of the inner wall of the insulated air pipe. Thus, the largest diameter of the supporting trunnion can be, for example, 99.5, 124.5, 159.5, or 199.9 mm.

In the description above, the end plate 1 14 that forms part of the mould device, is fastened to the fixing flange at the end of the casting mould by means of mounting bolts. Alternatively, the fastening of the end plate can be implemented by hinging one border of the end plate onto the end of the casting mould and by providing the end plate with a quick-release fastener, by means of which the end plate can be locked onto the casting mould. In this case, the end plates are not provided with any fixing holes. Furthermore, instead of four filling holes, the end plates can be provided with another number of filling holes. Preferably, there is only one filling hole, and in the first end plate only. In this case, the second end plate is provided with air holes which are smaller in size than the filling hole and through which air can escape the casting mould. Advantageously, two air holes are provided, having a diameter of 5 mm.

Figures 2a to 2d illustrate steps of a method for manufacturing an insulated air pipe according to the invention in a series of images. The mould device shown in Fig. 1 is applied in the method. An insulated air duct to be made by the method comprises an inner wall 20 and an outer wall 30. In the method, a prefabricated metal air duct, a so-called spiral-weld pipe, is used as the inner wall, and the outer wall is formed of a thin aluminium foil. A hose that is open at both ends is formed of the aluminium foil (Fig. 2a). The hose is dimensioned so that the perimeter of its cross section is slightly smaller than the perimeter of the cross section of the mould surface 104 of the casting mould 100. The length of the hose-like outer wall is selected to be slightly greater than the length of the casting mould, that is, the distance between the first and second openings of the casting mould.

The outer wall 30 is fitted in the casting mould via the first or the second opening 106, 108, and the first end of the outer wall is fastened to the brim of the first opening of the casting mould, and the second end of the outer wall is fastened to the brim of the second opening of the casting mould. After this, the rigid metal inner wall 20 is fitted inside the outer wall. Alternatively, the outer wall can be fitted around the inner wall first, after which both walls are inserted in the casting mould simultaneously. It is also possible to fit the inner wall inside the casting mould first and then to "pull" the outer wall onto the inner wall. The inner wall is also dimensioned to be substantially equal in length with the casting mould 100 (Fig. 2b). After fitting the inner wall 20 in the casting mould 100, it is supported to be immobile with respect to the casting mould. The inner wall is supported by fitting the end plates 1 14 against the first and the second openings 106, 108 of the casting mould 100 in such a way that the supporting trunnions 1 16 are placed inside the ends of the inner wall. The end plates are fastened to the casting mould by quick-release fasteners or mounting nuts 122 which are fitted to extend through the mounting holes in the collar and the mounting holes in the rim of the end plate. In this way, the inner wall is supported at both ends to the supporting trunnion 1 16 which prevents the inner wall from moving with respect to the casting mould (Fig. 2c). The filling holes 120 in the end plates will now open to the space between the inner wall and the outer wall.

Next, polyurethane is dispensed via the filling hole or filling holes 120 in the end plate 1 14 into the space between the inner wall and the outer wall, and is allowed to expand and to substantially fill up the space between the inner wall 20 and the outer wall 30. When the foamed polyurethane dispensed between the inner wall and the outer wall expands, it forces the outer wall 30 to be pressed against the mould surface of the casting mould 100. Thus, the outer wall 30 of the thermally insulated air duct assumes the shape of the mould surface of the casting mould. The expanded polyurethane adheres to the inner wall and the outer wall, and excess polyurethane can escape the casting mould via the filling holes or the air holes. The polyurethane is allowed to cure for a suitable length of time. After the curing, a rigid layer of thermal insulation 40 is provided between the inner wall and the outer wall.

Finally, the insulated air duct consisting of the inner wall, the outer wall and the thermal insulation layer is removed from the casting mould 100. For removing the air duct, the end plates are first removed from the front of the openings 106, 108. After that, the insulated air duct is simply pulled and/or pushed out of the casting mould 100, either via the first opening 106 or via the second opening 108. For pushing out, a rod or a bar can be used, which is inserted in the first opening of the casting mould, whereby it will push out the air duct from the second opening. For pulling out, it is possible to use a rope 90 or a wire that is passed through the pipe delimited by the inner wall 20 of the air duct and anchored at its first end to the first end of the inner wall. The air duct can thus be removed from the casting mould by pulling at the second end of the rope 90 (Fig. 2d).

In an advantageous embodiment of the method according to the invention, four narrow metal straps 50 are provided against the inner surface of the outer wall 30 of the air duct (Fig. 2d), extending from the first opening 106 of the casting mould to the second opening 108 of the casting mould. The straps are placed at substantially regular intervals at quarter points of the perimeter of the inner surface of the outer wall. The straps are pulled tight, and their both ends are squeezed be- tween the end plate 1 14 and the collar 1 10 of the casting mould. The straps are placed in the casting mould before the dispensing of the thermal insulation material. By means of the tight straps, the outer wall can be pressed against the mould surface of the casting mould, or close to it, so that an open space is formed between the inner wall 20 and the outer wall 30, for dispensing the thermal insulation material. After the curing of the thermal insulation, the strap constitutes in the air duct, immediately below the outer wall, a strip, that is resistant to tensile stress. The strap is adhered over its whole length to the thermal insulation layer, so that it will remain stable between the outer wall and the outer surface of the thermal insulation layer. It is obvious that the number of straps can naturally be different from four, for example 1 , 2, 3, 5, or 6.

Figure 3a shows an exemplary cross-sectional view of a joint between two thermally insulated air ducts according to the invention, and Fig. 3b shows a band 80 used in the joint. A conventional tubular sleeve joint piece 60 is mounted in the joint, its first end being fitted inside the inner wall of the first air duct and its second end being fitted inside the inner wall of the second air duct. In the joint, the ends of the air ducts to be connected are pressed as close to each other as possible, whereby a gap of about 10 mm is left between the end faces of their insulation layers

The gap formed in the joint is covered by an annular band 80 (Fig. 3b) whose first rim extends onto the outer wall of the first air duct and whose second rim extends onto the outer wall of the second air duct. Thus, the band covers the whole gap left under it. The band is made of thin, bendable sheet metal. Both rims of the strap are provided with triangular spikes 82 extending inwards from the plane of the band (Fig. 3b) and penetrating into the thermal insulation layer 40 when the strap is installed. The spikes are used as locking elements to prevent the air ducts from being pulled apart. Each end of the band is provided with an end part 84 bending at a right angle. Each end part is provided with two holes which are aligned with the respective holes in the opposite end part of the band. The end parts are fastened to each other and the band is tightened by screws or bolts to be driven in the locking holes. The locking holes can be provided with inner threads. Moreover, the wall of the band is provided with evenly spaced through holes 86, via which joint sealing foam 70 can be extruded into the gap covered by the band. Excess joint sealing foam extruded in the gap will exit the gap through the holes. If metal straps 50 are provided under the outer wall 30 in the air ducts to be connected, the band 80 can be fastened at its rims to the ends of the straps by screws 90. Fas- tening the straps at their ends to the band will significantly increase the tensile and bending strength of the joint.

Some advantageous embodiments of the method and the air duct according to the invention have been described above. The invention is not limited to the solutions described above, but the inventive idea can be applied in different ways within the scope of the claims.